Fibered metal powders

ABSTRACT

HARD METAL POWDER COMPACTS ARE SINTERED AND IMPREGNATED WITH A SOFTER METAL. THE COMPACTS ARE REDUCED TO ROD, WIRE OF SHEET. IN THE PROCESS FINE FIBERS OF THE HARD METAL POWDER ARE FORMED.

8- 1974 R. W. DOUGLASS ,8*?,65

FIBERED METAL POWDERS Original Filed Bax-ch 29, 1967 2 Sheets-Sheet 1POWDERS OF FIRST METAL vAOuuM IMPREGNATE MOLTEN WITH SECOND METAL ECONDMETAL I g'gfi; 'fgg'g ELONGATE TO ROO' A V I I ORAw TO C ROLL TO B wIRESHEET II DIFFUSION D REACTION (2) USE As V LEACH OUT COMPOSITE SECONDMETAL I I 1 USE As USE AS METAL FELT COMPOSITE RE IMPREGNATE E r V (b)SEPARATE DIFFUSION A FIBERS REACTION Aug. 6, 1 7Q w. DOUGLASS FIBEREDMETAL POWDERS Original Filed Each 29, 1967 2 Sheets-Sheet a I NVENTORWARD W 0045 atent Patented Aug. 6, 1974 1m. (:1. 1122f 3/26 U.S. c1.29-192 1 Clalm ABSTRACT OF THE DISCLOSURE Hard metal powder compacts aresintered and impregnated with a softer metal. The compacts are reducedto rod, wire or sheet. In the process fine fibers of the hard metalpowder are formed.

This application is a division of my earlier application, Ser. No.807,129, filed Mar. 13, 1969, now abandoned, which was in turn acontinuation of my application Ser. No. 626,773, filed Mar. 29, 1967,now abandoned. Other related copending applications are Ser. No. 74,962,filed Sept. 24, 1970, now US. Pat. 3,729,794, Ser. No. 869,404, filedMar. 13, 1969, now US. Pat. 3,681,063, as a division of 626,773, Ser.No. 839,024, filed July 3, 1969, now abandoned, as a division andcontinuation-in-part of Ser. No. 626,773 and 807,129 and 869,404, nowUS. Pat. 3,681,063, and Ser. No. 196,812, filed Nov. 8, 1971, now US.Pat. 3,742,369, as a division of said Ser. No. 839,024, now abandoned,Ser. No. 199,065, filed Nov. 15, 1971 as a continuation of Ser. No.839,024, now US. Pat. 3,740,- 834, also is a related copendingapplication.

The present invention relates to metal fibers or filaments useful for avariety of purposes including capacitors, filters, structuralreinforcement. The fibers are particularly of the class of hard metalshaving high strength and high temperature use capability (having atleast 50% room temperature strength at 500 C.) and extraordinarily smalldiameter as on the order of a micron or less, while having continuouslength of several times diameter and as high as ten inches.

The invention relates to such filaments as separate entities, in loosebundles (i.e. a metal felt) or as incorporated in reinforced matricesand to the process of making them.

BACKGROUND Metal felts and fine metal wires or fibers or filaments usedin such felts are known in the art as indicated in Pats. 2,903,787 and3,178,280. These felts are made from standard cold reduced metal wireswhich are limited to minimum diameters on the order of .001.010 inchesor less by the inherent vulnerabilitites of standard wire drawingprocesses or from shavings from metal blocks which are characterized bymany surface defects. Much finer wires can be made by extrusion asindicated in Pat. 3,199,- 331 to Allen. But production by this processis substantially limited as a practical matter to low melting metals andalloys (e.g. tin). Other prior art of interest is Buehler, US. Pat.3,124,455 and the Speidel, Levy and Wulfi' work cited below.

The present invention involves as a principal object the production ofmetal fibers of sub-micron size by a new process which is capable ofbeing used with high temperature metals such as tantalum.

It is a further object of the invention to provide an economical methodof making metal fibers on the order of 10 microns or less, andpreferably sub-micron, in diameter with a single series of processingsteps; i.e. free of the expensive supplementary or recycling processinginvolved, for instance, in Speidel, US. Pat. 3,256,118, Levy, US. Pat.3,029,496 and Wulff, January 1966 Journal of Applied Physics, p. 5.

It is a further object of the invention to provide work hardened fibersby a production process free of the need for intermediate anneals asrequired in the above patents of Allen and Levy, and for use incomposites providing a high degree of work hardening in final productform, with or without a final low anneal for stress relief of the matrixonly.

Other objects, features and advantages of the present invention will inpart be obvious and will in part appear hereinafter.

DESCRIPTION The invention is now described with respect to typicalspecific embodiments thereof and with reference to the accompanyingdrawings wherein:

FIG. 1 is a block diagram of the process of the invention.

FIG. 2 is a copy of a photomicrograph of a composite according to theinvention.

FIG. 3 is a copy of a photomicrograph of a metal felt according to theinvention.

The fibers of the invention are made and used by the following processdescribed with reference to FIG. 1 which is a block diagram of theprocess. First, powders of the metal to be fibered are obtained. Themetal may be any of tantalum, niobium, molybdenum, tungsten, iron orstainless steels, titanium, nickel, aluminum, chromium, beryllium,magnesium oxide, titanium hydride and fabricable aluminides andsilicides. The invention would also be of particular utility anddistinctly advantageous benefit in fibering other hard metal elements,compounds or alloys which have softening temperatures in excess of about1000 C. The starting powder size is variable depending upon subsequentprocessing and reactivity of the powders. The invention has beenpracticed successfully for instance with tantalum powders as large asminus mesh and as small as a few microns diameter. The powder isconsolidated into a compact by pressing and sintering or sintering in amold. Then a melt of a second metal is provided in vacuum or inertatmosphere and the powder compact of the first metal is impregnated bydipping in the melt. During both the sintering and impregnating stepsthe compact is degassed and purified to enhance its wettability andductility.

The second metal may be any of aluminum, copper, nickel, Woods metal,tin, indium, mercury, or any other metal which meets the followingcriteria with respect to the first metal under the conditions ofimpregnation:

(1) readily wet the skeleton structure of the sintered compact of thefirst metal; (2) not alloy extensively with the first metal;

(3) have similar hardness and fabrication characteristics to the extentnecessary for co-working;

(4) be easily removable from the compact by chemical or thermal means.

The impregnated compact is then worked down to an elongated rod form orthe like e.g. plate or cylinder (round or rectangular cross section) byswaging or forging. During this process the adjacent particles of hardmetal in the compact begin to form long fibers within the matrix of thesecond metal.

At this point, the rod or cylinder or plate may be used or fabricatedinto a useful product in any of the following ways:

A(1)-Removing the matrix metal and (a) using directly as a filter orwith further fabrication as a capacitor (b) separating out individualfibers (c) re-impregnating the fibered article A-(2)--Using the roddirectly as a composite structural element B-Rolling the rod to sheetprior to 1) or (2) above CDrawing the rod to wire prior to (1) or (2)above DHeating the rod for diffusion reaction between the hard metalfibers and the matrix metal prior to (l) or (2) above.

Several permutations of the foregoing can be made. For instance a rodcan be drawn for several passes before rolling. A wire or sheet can beheated for diffusion reaction. Similarly a re-impregnated article can beused as a composite, with or without a diffusion reaction, or releached.With diffusion reactions, fibers of alloys or compounds can be formedeven though such alloys are too brittle to be fibered directly. Anotheralternative in the scope of the invention is to form a loose fiberbundle or separate fiber ((a) or (b) above) and expose it to anoxidizing or nitriding atmosphere. In this way fibers of aluminum oxideor aluminum nitride can be made for use in reinforced compositestructures. Also fibers of tantalum or niobium nitride can be made foruse as superconductors. In these applications it is of special interestthat the fiber diameters are so small as to favor the formation of theabove compounds in single crystal form which is especially desirable.

The fibers of the invention are characterized in that each fiber isderived from a single powder particle and its length is dependent on thedegree of diameter reduction. For instance, an 8 micron diameter powderparticle fibered to 0.1 microns diameter will have a length of about oneinch, a 30 micron diameter particle fibered to 0.1 microns diameter willhave a length of about seventy inches. Further cold working to finerfiber diameters would increase the length. In most applications of theinvention, useful fibers will have a length of ten times the diameter ofthe fiber or longer (as high as 10 times for extreme cases).

The felts of the invention are characterized by substantialcross-linking by metallurgical bonds between tangentially contactingfibers corresponding in part to the bonds between powders in theoriginal powder compact skeleton and corresponding in part to new bondsformed during cold working the impregnated compact down to an elongatedarticle, the new bonds being essentially an extension or stretching outof the old bonds.

FIG. 2 shows longitudinal section photomicrograph of a composite in theform of a wire of .039 inch diameter at 133 times magnification. Thecomposite has elongated reinforcing tantalum fibers in a matrix ofcopper. The starting material for the fibered metal was coarse meltinggrade powder minus 12 and plus 60 mesh pressed at 18,000 p.s.i. andsintered at 2300 C. for one hour to produce a compact of 61% density.

FIG. 3 shows a longitudinal section photomicrograph of a tantalum metalfelt, encapsulated in a molding resin for microscope examination, at 266times magnification. The tantalum was made from nominal 8 microndiameter powders (minus mesh and plus 5 microns) which was consolidatedto a compact of about 50% density and then impregnated with copper andthen swaged to rod and rolled to sheet after which the copper wasleached out in a nitric acid bath. Upon leaching the metal feltballooned up to several times its original volume.

Fibers obtained from rod or wire are found to be essentially circular incross-section and fibers obtained from sheet are found to be rectangularin cross-section. The term diameter as used herein refers to diameter ofa circle or width of a rectangle.

The practice of the invention is further illustrated by the followingnon-limiting Examples.

EXAMPLE 1 A mold was filled with tantalum powder of about 8 micronnominal diameter (-100 mesh and plus 5 microns) and the powder wassintered in the mold at 1500 C. for 20 minutes to form a green compact.Then sintering was completed by removing the compact from the mold andheating at 2300 C. for one hour to complete consolidation of the powder.The density of the compact was 8.22 gms./cc. or 49.5% of theoreticaldensity. The compact was vacuum impregnated with copper by dipping in amolten copper bath at 117 0 C. for 5 minutes under a vacuum of about l0torr. The impregnated compact (.35 inches diameter by 4 inches long) wasenclosed in an iron pipe and then swaged to .125 inches diameter. Thejacket was removed and the rod was then further swaged to .080 inchesdiameter. After swaging, the rod was then leached in nitric acid toremove the copper. The leached compact left a bundle of interwoventantalum fibers in the form of a felt.

Thi metal felt was rinsed and removed from the leach bath. The felt wasanodized and formed into a capacitor anode and tested for capacitorproperties in a wet electrolyte. The formation voltage was 200 volts andthe capacitance was 30.6 microfarads and on a specific weight basis 6120microfarad-volts per gram. The felt had a dissipation factor of 32.19%making it an over-all operable capacitor anode.

EXAMPLE 2 Tantalum felts were made as in Example 1 but with thedifference that the compact was rolled to .010 inch thick sheet beforeleaching. The felt exhibited a vigorous swelling up with a volumeincrease and density decrease of 5-10 times during leaching and floatedon the leaching bath. A capacitor formed from the felt at volts had 7965microfarad-volts per gram specific capacitance.

EXAMPLE 3 Felts were made as in Examples 1 and 2 with the differencethat consolidation of the tantalum powder was accomplished by pressingat 18,000 p.s.i. and then sintering at 2250 C. for one hour and thatsome rods were drawn to wire. Densities of 60-80% of theoretical wereobtained in the original compact. Upon leaching the final compositearticle of this type, the felt did not swell up. However, high values ofcapacitance were still obtained indicating substantial formation of newsurface as in Examples 1 and 2 (surface enhancement of about 2.5 times).

EXAMPLE 4 Several fibers from the felts of Examples 1 and 2 wereencapsulated in epoxy resin and measured to yield an individual fiberdiameter indication of .0002 cm. diameter. The Example 2 fibers were 5to 10 times as long as the diameter of the fiber; the Example 1 fiberswere continuous over much longer lengths.

EXAMPLE 5 Several compacts made essentially as in Examples 1 and 2 wererolled or drawn to the final sizes indicated below for testing of theircomposite material properties. These tantalum reinforced coppercomposites were in the form of .020 inch diameter wire and as .010 inchthick sheet, both as worked and after being heated (350 C. for 1 hour toanneal the copper). The results for these specimens and for comparison,the properties of tantalum and copper, per se, are given in Table 1:

TABLE 1 Example 5 Ultimate tensile sample: strength, p.s.i.

(a) .01.020 inch diameter wire as worked 160,000195,000 (b) Wire withstress relief 150,000-172,000

(c) Sheet, as worked 99,000-127,000

(d) Sheet, stress relieved 93,000 (e) Pure tantalum, as worked (.005 and.015 inch thick sheet) 104,000-116,000 (f) Pure copper, as worked (.005and .015 inch thick sheet) 59,000-60,500

EXAMPLE 6 A molybdenum-copper composite was made and tested in the samemanner as the tantalum-copper composites of Example 5 and formed into.06 and .08 in. wire which displayed ultimate tensile strengths of81,700 and 108,000 p.s.i., respectively.

EXAMPLE 7 Tantalum felts made as in Examples 4 and 2 were tested fortensile strength after leaching out the copper. The results are in Table2.

TABLE 2 Ultimate tensile Example sample: strength, p.s.i. (a) .01 in.sheet 114,700 (b) .04 in. wire 90,000

EXAMPLE 8 Iron powder of 270 mesh was mold sintered at 800 C. for 20minutes and then finally sintered at 1150 C. for 1 hour to a density of3.45 grams per cc. (45% theoretical) impregnated as above and worked to.025 inch wire and leached to form a fibrous bundle of iron fibers .0015cm. diameter, quite continuous and having a surface layer of copper-ironalloy overlaid by residual copper but with a substantial core of pureiron in the fibers.

' EXAMPLE 9 Before leaching, the iron-copper composite Wire of Example 8was tested for tensile strength and this was found to be 160,000 p.s.i.

EXAMPLE 10 Leaching experiments were conducted and a solution of fiveparts ammonium hydroxide in one part hydrogen peroxide was found to besuperior to nitric acid for selectively leaching copper from the iron tofree the iron fibers from the composites.

The best mode of using the invention is believed to be selection of atantalum-copper pair to produce a tantalum felt suitable for use as acapacitor anode. In addition to the above indicated advantages of easeof processing, surface enhancement and work hardening it is a furtheruseful advantage of the invention that it may be practiced if desired,with relatively coarse melting grade tantalum powder in the originalcompact rather than the conventional fine grain capacitor grade powderand the desired surface area increase can be obtained in thefiberpowder. A further useful aspect of the invention is the abovedescribed feature of swelling When the original compact is made in lowdensity (4060% theoretical) and/or when a high degree of working is putinto the composite. The swelling of the metal felt, when utilized makesit easier to refill the felt with an anodizing medium and electrolyte.

The extension to other species of the above advantages and variations inprocessing and still other advantages and variations will be obvious tothose skilled in the art from the description herein. For instance, aniobium-tin pair could be utilized to obtain interconnected niobiumfibers in a tin matrix with a better degree of interconnection betweenfibers than is obtainable in the process of the above described Speidelpatent. Then .the composite could be heated for diffusion reaction toform a niobium stannide superconductor subsequent to which residual tinwould be leached out and replaced with copper by re-impregnation toprovide a higher conductivity matrix for electrical stability of thesuperconductor.

A high degree of control of the final product is obtainable. Forinstance, use of coarse melt grade powders or low density consolidationof the original compact (40- 60%) tend to limit the number of cross-linkbonds formed between fibers thereby enhancing the swelling up of fibersupon leaching the matrix metal and enhancing the ease of separation offibers.

For superconductor applications it is particularly desirable to use afine grain powder and form the original compact to a higher density forforming maximum crosslinks between fibers.

Still other applications within the scope of the present invention willbe apparent to those skilled in the art when aided by the foregoingdescription. The description is therefore intended to be read asillustrative and not in a limiting sense.

What is claimed is:

1. A felt of refractory metal fibers which are interconnected to eachother by spaced metallurgical bond crosslinks,

as produced by impregnating a sintered refractory metal powder porouscompact having powder-to-powder metallurgical bond cross-links betweenpowder particles with a second metal in fluid form solidifying thesecond metal working the impregnated compact down to an elongatedarticle to elongate the metal powders to fibers and to elongate thebonds and then removing the second metal,

to thereby produce an elongated felt product having a characteristicdirection of elongation with interconnected fibers being therein whichare similarly elongated with each fiber being derived from a singlepowder particle and the cross-links being derived in part from originalpowder cross-links,

the felt having enhanced internal surface area compared to the originalporous compact.

References Cited UNITED STATES PATENTS 2,627,531 2/1953 Vogt 136-202,972,554 2/1961 Muskat et al. 11776 3,029,496 4/1962 Levi 29419 X3,127,668 4/1964 Troy 29182 3,254,189 5/1966 Evaniscko, et al. 200166 C3,310,387 3/1967 Sump et al 29-182 X 3,087,233 4/1963 Turnbull 291823,337,337 8/1967 Weeton et al 204 ALLEN B. CURTIS, Primary Examiner O.F. CRUTCHFIELD, Assistant Examiner

